This invention relates to diesel fuel injectors and fuel injection pumps. The invention is applicable to unit injectors used on locomotive, automotive, marine and stationary engines, in which the pump, nozzle and holder assembly are a single unit. The invention is also applicable to injection systems in which the fuel is fed from the pump through tubing to a separate nozzle-and-holder assembly.
A known type of fuel injector for diesel engines comprises a fuel pump and an injection nozzle associated with the fuel pump. The fuel pump includes a pump cylinder and a pump plunger reciprocatable in the cylinder. The cylinder and plunger together define a pump chamber open at one end for the discharge of fuel during a pump stroke and for fuel intake during a suction stroke of the plunger. The injection nozzle is associated with a valve body having a spray outlet at one end for the discharge of fuel at the nozzle tip. The nozzle valve is movable in the nozzle body between open and closed positions to control flow through the spray outlet. The valve is spring-biased to closed position and openable when such discharge of fuel during a pump stroke reaches a given level of pressure. The valve then remains open until pressure drops to a closing pressure somewhat below the opening pressure. The closing pressure is below the opening pressure because the valve face area subject to opening pressures is somewhat greater when the valve is open and unseated than when it is closed and seated.
Fuel is supplied to the pump and excess fuel is returned from the pump to reservoir through low pressure passages communicating with the pump chamber. The low pressure passages constitute fill and spill passages. The inlet port area is large enough to fill the pump chamber under the highest engine operating speeds. The flow area of the spill port is large enough that the fuel is spilled back into the fuel supply ducting of the low pressure supply system (or into the return to reservoir, which for present purposes counts as part of the fuel supply ducting) at a rate high enough to prevent the discharge of fuel, resulting from the pump stroke, from reaching the given pressure at which the nozzle valve opens to commence fuel injection, or from remaining above the somewhat lower given pressure at which the open injection valve closes. The ports close and then open during each stroke of the pump plunger to thereby establish, between the closing and opening, the portion of the pump stroke during which such discharge of fuel occurs at pressures above the closing pressure of the injection nozzle.
In purely mechanical injectors, the spill referred to is wholly mechanical, generally in the form of edges and cut-outs formed on the pump plunger which interact with ports opening into the pump bore from the low pressure passages. In other injectors, the spill valving is controlled by a solenoid which opens and shuts a spill passage valve.
Fuel injection, that is, delivery of fuel to the injection nozzle downstream of the plunger chamber at a high enough pressure to cause the nozzle valve to open, occurs during that part of the pump stroke during which the spill passages are closed.
The initial rate of fuel injection has a profound influence on the maximum combustion pressure and temperature generated in a diesel engine combustion chamber during engine operation. When combustion pressure and temperature are elevated above certain limits, nitrogen is oxidized to form nitrous oxide. Ignition delay is the principal reason for generation of such excessively high pressure and temperature. Improved ignition quality of fuel and higher compression pressures can reduce the ignition delay period, but there is a limit to the improvement that can be achieved with improved fuel quality which also carries a cost penalty. Higher compression pressures also have the adverse effect of increasing maximum combustion pressure which in turn tends to increase the formation of nitrous oxide.
Various proposals have been made to deliver injected fuel at a lower rate during the early part of the injection portion of the pump stroke corresponding to the ignition delay period. For example, it has been attempted to deliver fuel at an initially reduced rate by using a two stage lift-cam whereby the initial portion of the cam lift is limited to produce a fixed quantity of fuel delivery by the plunger and then the cam lift ceases for a small period, or slows down, and then resumes its lift at the normal rapid rate to complete the plunger stroke. This two-stage lift method has not been successful because the initial pressure wave generated at port closing is a function of engine speed and injection is inconsistent in the low and intermediate engine speed ranges.
Another previous method has used a separate small plunger to inject a small pilot quantity of fuel preceding the delivery by the main plunger of the main quantity of fuel required by the engine to develop the power required. This is a mechanically complicated and relatively costly system and has not been successful.
It has also been known in the prior art to provide auxiliary spill porting for a reduced rate of fuel feed in the early part of the injection portion of the plunger stroke, but such arrangements were intended to minimize initial injection pressure and were not successful. An example is seen in U.S. Pat. No. 2,513,883 to J. F. Male. It has also been known to use auxiliary spill porting arrangements effective at varying proportions of the injection portion of the feed stroke, as for example in U.S. Pat. No. 4,741,314 to Hofer in which auxiliary porting is arranged so there is a declining duration of leakage as the engine load increases in a straight line relationship with load such that maximum duration of leakage is at idle and there is zero duration of leakage at full load.
Other spill porting systems have been proposed as for example in my U.S. Pat. Nos. 5,870,996 and 6,009,850. These systems are effective, but may be too costly or difficult to fabricate in smaller injector sizes.
Another alternative proposed in the prior art is subtract-then-add-back porting, wherein instead of spilling some of the fuel during the injection portion of the plunger stroke, a portion of the fuel is temporarily diverted from the pump chamber to a fuel subtraction chamber and then during the return stroke of the plunger is returned (added back) from the subtraction chamber to or toward the pump chamber. An example is seen in U.S. Pat. No. 4,811,715 to Djordjevic et al. in which the volume of the fuel subtraction chamber 134 expands and contracts as the pin 128 is forced up into the accumulator chamber 136 and back down again by varying pressures according to the load and speed conditions of the injection pump. However, in this patent, the subtracted fuel is not contained entirely in the subtraction chamber, but in very small part leaks back and forth past the pin 128 into and out of the accumulator chamber 136. Fuel can start to be subtracted from the system before injection begins because the pressure in the accumulator chamber can drop below the nozzle opening pressure during the residual pressure phase of the injection cycle (which is much longer in duration than the injection phase), depending upon the clearance between the plunger and its guide. Also, the condition can vary from injector to injector depending upon the respective clearances of the plungers in the injectors.
British Patent 634,030 also shows a form of subtract-then-add-back porting. Although the patent does not describe itself in those terms, it does contain a fuel subtraction chamber m (not denominated as such) to which a portion of the fuel is diverted as the pump stroke begins and from which the diverted fuel is returned (added back) during the return stroke of the pump. However, the operation of the disclosed device is necessarily such that injection is interrupted and then resumed during the pump stroke (although interruption is not mentioned in the patent description). Such interruption is in part due to lack of constraint on flow into the chamber m as it fills. (Passage n is unrestricted by any control orifice, and the cross-sectional areas of the passages n and g are of comparable size, assuring that as the chamber m rapidly fills and until plunger k reaches the limit of its travel, flow into the chamber m will be at a rate not substantially less than the flow rate through the nozzle orifices c that obtained prior to filling of chamber m, thus abruptly dropping pressure in nozzle chamber e so that the spring i closes the nozzle valve to interrupt injection until such time as plunger k reaches the limit of its travel.)
U.S. Pat. No. 4,681,080 to Schukoff also shows a form of subtract-then-add-back porting. Schukoff""s arrangement is very similar to that of the above-discussed British patent. Schukoff explicitly describes how injection is interrupted and then resumed during the pump stroke.
Another example of subtract-then-add-back porting is shown in K. P. Mayer""s SAE Paper No.841288, 1984. The device shown in FIG. 3 of Mayer includes a piston within the fuel subtraction chamber formed in the bore of Mayer""s xe2x80x9cbarrelxe2x80x9d member. After referring to and describing the device shown in his FIG. 3 as a xe2x80x9csplit injection devicexe2x80x9d wherein there is a separation between initial and main injection, Mayer says that xe2x80x9c[a]t medium and high engine speeds it is necessary to avoid a separation between the initial and the main injection because of the resulting increase in smoke. At these operating conditions the split injection device is therefore adjusted to only briefly slow down the initial rate of fuel discharge as shown in FIG. 4.xe2x80x9d However, how such adjustment is made is neither described nor apparent. Nor is it apparent how, if such adjustment is somehow made, the split injection device is then capable of being readjusted to perform as a split injection device at operating conditions not requiring separation between initial and main injection.
As indicated in Mayer, split injectionxe2x80x94initial (pilot) injection at a reduced rate followed by a brief interruption and then main injectionxe2x80x94has been associated with an increase in smoke under full or high load conditions. Conversely, monolithic injectionxe2x80x94a reduced-rate-of-injection phase followed without interruption by a main injection phasexe2x80x94has been associated with reduction of overall emissions. An important object of the present invention is to provide improved monolithic injection under varying loads and operating speeds of the injection system.
The present invention provides subtract-then-add-back porting from the pump chamber through a flow-subtraction control orifice to a subtraction chamber in which flow into the subtraction chamber is governed by a control orifice of fixed cross-sectional area bearing a predetermined ratio to the total cross-sectional area of the injection nozzle orifices such that monolithic injection occurs at both low and high load conditions at all operating speeds.
During injection, the flow-subtraction piston is moved from a minimum-volume position to a maximum-volume position so as to contain, on the front or face side of the piston, all subtracted fuel fed into the subtraction chamber. A passageway is connected to the fuel supply ducting of the injector""s low-pressure fuel supply system and opens into the space behind the piston to provide for dumping fuel that is behind the piston during filling of the subtraction chamber on the face side of the piston. Significantly, the minimum cross-section of this passageway is sufficient to also allow inlet pressure in the fuel supply ducting to help the piston spring return the piston to its minimum-volume position and thereby help return (add back) the subtracted fuel to or toward the pump chamber during the return stroke of the pump.
The minimum-volume position of the flow-subtraction piston is also a closed or seated position characterized by a relatively high opening-pressure requirement. The intermediate positions of the flow-subtraction piston are unseated positions characterized by relatively lower translating-pressure requirements. Thus, during each pump stroke subtractive flow through the flow-subtraction control orifice is delayed until the relatively high opening-pressure requirement is met. Thereafter the rate of subtractive flow is governed by the cross-sectional area of the flow-subtraction control orifice and the injection pressure prevailing at the area of the control orifice. Subtractive flow continues until the flow-subtraction piston comes up against a fixed stop, and injection at a higher flow rate (due to the termination of subtractive flow) continues immediately thereafter, without any interruption of injection between the reduced-rate-of-injection phase and the main injection phase at a higher rate. Such monolithic injection is maintained at both low and high load conditions.
The result is monolithic or continuous injection during both reduced-rate-of-injection and main injection phases, precise timing of the initiation of subtractive flow relative to the initiation of injection, precise proportioning between subtractive flow and flow through the injection nozzle orifices during the interval during which the flow-subtraction piston moves from its minimum volume position to its maximum-volume position, and robust return of subtracted fuel to or toward the pump chamber during the return stroke of the pump plunger, all accomplished under both low and high load conditions and varying speed conditions.
The invention thereby provides improved means to produce monolithic reduced-initial-rate fuel injection by using subtract-then-add-back porting. Part of the fuel delivered by the pump plunger is diverted simultaneously with or immediately following the beginning of fuel injection through the nozzle orifices and continuing during a reduced-rate-of-injection phase which generally corresponds to the engine ignition delay period. In some cases it may be desirable to continue diversion for a brief interval beyond ignition as may be found necessary to optimize combustion and the reduction of nitrous oxide formation during combustion.
The invention will be more readily and fully understood from the following detailed description and the accompanying drawings.